Thin film deposition techniques are widely used in the manufacturing of micro-devices to form a coating on a workpiece that closely conforms to the surface topography. The size of the individual components in the devices is constantly decreasing, and the number of layers in the devices is increasing. As a result, the density of components and the aspect ratios of depressions (e.g., the ratio of the depth to the size of the opening) are increasing. The size of workpieces is also increasing to provide more real estate for forming more dies on a single workpiece. Many fabricators, for example, are transitioning from 200 mm to 300 mm workpieces, and even larger workpieces will likely be used in the future. Thin film deposition techniques accordingly strive to produce highly uniform conformal layers that cover the sidewalls, bottoms, and corners in deep depressions that have very small openings.
One widely used thin film deposition technique is Chemical Vapor Deposition (CVD). In a CVD system, one or more precursors that are capable of reacting to form a solid thin film are mixed in a gas or vapor state, and then the precursor mixture is presented to the surface of the workpiece. The surface of the workpiece catalyzes the reaction between the precursors to form a thin solid film at the workpiece surface. A common way to catalyze the reaction at the surface of the workpiece is to heat the workpiece to a temperature that causes the reaction.
Although CVD techniques are useful in many applications, they also have several drawbacks. For example, if the precursors are not highly reactive, then it is often necessary to heat the workpiece to a temperature that catalyzes the reaction between the precursors. Heating the workpiece, however, can be detrimental to the structures and other materials already formed on the workpiece. Implanted or doped materials, for example, can migrate within the silicon substrate at higher temperatures. On the other hand, if more reactive precursors are used so that the workpiece temperature can be lower, then reactions may occur prematurely in the gas phase before reaching the substrate. This is undesirable because the film quality and uniformity may suffer, and also because it limits the types of precursors that can be used.
One conventional system to prevent premature reactions injects the precursors into the reaction chamber through separate ports. For example, each port of a showerhead can be coupled to a dedicated gas line for a single gas. Systems that present the precursors through dedicated ports proximate to the surface of the workpiece, however, may not sufficiently mix the precursors. Accordingly, the precursors may not react properly to form a uniform, thin solid film on the workpiece surface. Furthermore, conventional systems also have a jetting effect that produces a higher deposition rate directly below the ports. Thus, conventional CVD systems may not be appropriate for many thin film deposition applications.
Atomic Layer Deposition (ALD) is another thin film deposition technique. FIGS. 1A and 1B schematically illustrate the basic operation of ALD processes. Referring to FIG. 1A, a layer of gas molecules Ax coats the surface of a workpiece W. The layer of Ax molecules is formed by exposing the workpiece W to a precursor gas containing Ax molecules, and then purging the chamber with a purge gas to remove excess Ax molecules. This process can form a monolayer of Ax molecules on the surface of the workpiece W because the Ax molecules at the surface are held in place during the purge cycle by physical adsorption forces at moderate temperatures or chemisorption forces at higher temperatures. The layer of Ax molecules is then exposed to another precursor gas containing By molecules. The Ax molecules react with the By molecules to form an extremely thin layer of solid material on the workpiece W. The chamber is then purged again with a purge gas to remove excess By molecules.
FIG. 2 illustrates the stages of one cycle for forming a thin solid layer using ALD techniques. A typical cycle includes (a) exposing the workpiece to the first precursor Ax, (b) purging excess Ax molecules, (c) exposing the workpiece to the second precursor By, and then (d) purging excess By molecules. In actual processing, several cycles are repeated to build a thin film on a workpiece having the desired thickness. For example, each cycle may form a layer having a thickness of approximately 0.5-1.0 Å, and thus it takes approximately 60-120 cycles to form a solid layer having a thickness of approximately 60 Å.
FIG. 3 schematically illustrates an ALD reactor 10 having a reaction chamber 20 coupled to a gas supply 30 and a vacuum 40. The reactor 10 also includes a lid 80 coupled to the reaction chamber 20, a heater 50 that supports the workpiece W, and a gas distributor 60 in the reaction chamber 20. The gas distributor 60 includes a plenum 62 operatively coupled to the gas supply 30 and a distributor plate 70 having a plurality of holes 72. In operation, the heater 50 heats the workpiece W to a desired temperature, and the gas supply 30 selectively injects the first precursor Ax, the purge gas, and the second precursor By as shown above in FIG. 2. The vacuum 40 maintains a negative pressure in the chamber to draw the gases from the gas distributor 60 across the workpiece W and then through an outlet of the reaction chamber 20.
One drawback of ALD and CVD processing is that it is difficult to control the uniformity of the deposited films over a long period of time. For example, the individual precursors and/or the resulting material that is formed by the reaction between the precursor gases may build up on the components of the reaction chamber 20. Additionally, when the adsorption of the first precursor and/or the second precursor reaches a saturation point, the precursors will then begin to desorp into the gas flows. Such adsorption and desorption of the precursors continually changes the concentration of the first and second precursors in the chamber, which often adversely affects the quality of the film deposited onto the workpieces. As a result, the ALD and CVD reactors may need to be taken off-line for cleaning, which is time-consuming and reduces the throughput of a fabrication process. Therefore, there is a need to provide better control of the adsorption and desorption rates to achieve more consistent results throughout a run of workpieces and reduce downtime of ALD and CVD reactors.